Intentionally imbalancing propellers for performance and noise-shaping

ABSTRACT

Aerial vehicles may be operated with discrete sets of propellers, which may be selected for a specific purpose or on a specific basis. The discrete sets of propellers may be operated separately or in tandem with one another, and at varying power levels. For example, a set of propellers may be selected to optimize the thrust, lift, maneuverability or efficiency of an aerial vehicle based on a position or other operational characteristic of the aerial vehicle, or an environmental condition encountered by the aerial vehicle. At least one of the propellers may be statically or dynamically imbalanced, such that the propeller emits a predetermined sound during operation. A balanced propeller may be specifically modified to cause the aerial vehicle to emit the predetermined sound by changing one or more parameters of the balanced propeller and causing the balanced propeller to be statically or dynamically imbalanced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/975,563, filed Dec. 18, 2015, the contents of which are incorporatedby reference herein in their entirety.

BACKGROUND

Sound is kinetic energy released by vibrations of molecules in a medium,such as air. In industrial applications, sound may be generated in anynumber of ways or in response to any number of events. For example,sound may be generated in response to vibrations resulting from impactsor frictional contact between two or more bodies. Sound may also begenerated in response to vibrations resulting from the rotation of oneor more bodies such as shafts, e.g., by motors or other prime movers.Sound may be further generated in response to vibrations caused by fluidflow over one or more bodies. In essence, any movement of molecules, orcontact between molecules, that causes a vibration may result in theemission of sound at a pressure level or intensity, and at one or morefrequencies.

The use of unmanned aerial vehicles such as airplanes or helicoptershaving one or more propellers is increasingly common. Such vehicles mayinclude fixed-wing aircraft, or rotary wing aircraft such asquad-copters (e.g., a helicopter having four rotatable propellers),octo-copters (e.g., a helicopter having eight rotatable propellers) orother vertical take-off and landing (or VTOL) aircraft having one ormore propellers. Typically, each of the propellers is powered by one ormore rotating motors or other prime movers.

A propeller is statically balanced (or in static balance) when thepropeller remains at rest, and may remain in any position, when thepropeller is not powered. A propeller is dynamically balanced (or indynamic balance) when the propeller rotates evenly and withoutvibration. For example, a propeller that is statically balanced may bedynamically imbalanced, when the blades of the propeller have differentcenters of mass or gravity, or centers of mass or gravity that are notin common planes, such that centrifugal forces act on the blades indifferent planes and do not counteract one another. Conversely, where apropeller is dynamically balanced, centrifugal forces acting on theblades are equal to and counteract one another, and any vibrationsobserved should be minimal.

Traditionally, the balancing of propellers has been recognized one ofthe most important considerations of a properly operating aerialvehicle. For example, in aircraft having large propellers, vibrationsgenerated by propellers that are either statically or dynamicallyimbalanced have resulted in undue stresses to crankshafts or othercomponent parts. In aerial vehicles of all sizes, such vibrations mayresult in undesired or untenable noise levels within a vicinity of therotating aircraft.

An aerial vehicle is typically outfitted with a homogenous set ofpropellers that are balanced, both statically and dynamically, duringoperation. The propellers may be operated collectively or in groups. Forexample, a quad-copter having four propellers may operate each of thefour propellers during take-off or landing evolutions, where thequad-copter's lift capacity is preferably maximized. When thequad-copter is aloft at a desired altitude, and a maximum lift capacityis no longer desired, motors associated with one or more of thepropellers may be stopped for any reason, such as to preserve power orfuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are views of aspects of an aerial vehicle inaccordance with embodiments of the present disclosure.

FIGS. 2A through 2I are views of imbalanced propellers in accordancewith embodiments of the present disclosure.

FIG. 3 is a flow chart of one process for operating a propeller inaccordance with embodiments of the present disclosure.

FIGS. 4A and 4B are views of aspects of a balanced propeller and animbalanced propeller in accordance with embodiments of the presentdisclosure.

FIG. 5 is a flow chart of one process for operating an aerial vehicle inaccordance with embodiments of the present disclosure.

FIG. 6 is a view of aspects of one system for operating aerial vehiclesin accordance with embodiments of the present disclosure.

FIG. 7 is a flow chart of one process for operating an aerial vehicle inaccordance with embodiments of the present disclosure.

FIG. 8 is a view of aspects of an operating aerial vehicle in accordancewith embodiments of the present disclosure.

FIG. 9 is a block diagram of aspects of one system for operating anaerial vehicle in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

As is set forth in greater detail below, the present disclosure isdirected to operating aerial vehicles with discrete sets of propellers,including one or more imbalanced propellers. More specifically, thepresent disclosure is directed to selecting a complement of propellerson any basis and outfitting an aerial vehicle with the selectedcomplement of propellers. For example, one or more of the propellers ofthe complement may be selected on any basis, including but not limitedto any operational characteristics or environmental conditionsassociated with a mission of the aerial vehicle, or any sounds that maybe preferably emitted during operation. In some embodiments, an aerialvehicle may be outfitted with a set of propellers that are knowingly andintentionally out of balance, either statically or dynamically. Theaerial vehicle may then be operated with the complement of propellersoperating in any manner, e.g., alone or in any combination, in order tocomplete a mission, in accordance with a transit plan, or in order tocause the aerial vehicle to emit any kind or type of sound.

In some embodiments, an aerial vehicle may be outfitted with a discreteset of one or more propellers of a first type, or propellers having afirst operational attribute, and with a discrete set of one or morepropellers of a second type, or propellers having a second operationalattribute. During operation, power may be selectively applied to each ofmotors for rotating the propellers of the first set, or each of motorsfor rotating the propellers of the second set, as necessary, in order toexploit one or more characteristics or attributes of propellers of therespective types or having the respective operational attributes. Forexample, a first set of propellers may be configured to maximize one ormore of thrust, lift, maneuverability or efficiency, e.g., to conservepower and/or fuel, while a second set of propellers may be configuredbased on one or more acoustic considerations, e.g., to emit sound at apredetermined sound pressure level (or intensity) and/or at a frequency,or within a frequency spectrum (or distribution, pattern or band). Thepropellers of the first set and the propellers of the second set mayhave different radii or diameters, masses, blade lengths, blade widths,blade shapes or blade angles. The propellers may further have differentnumbers of blades, as necessary. Moreover, the first set of propellersand the second set of propellers may be selected on any operationalconsideration and/or environmental condition anticipated during apredetermined mission, including but not limited to a location of anorigin or a destination for a mission, a dimension or mass of a payload,a course or a speed to be followed during the mission, or anyanticipated temperatures, pressures, humidities, wind speeds ordirections, weather events, measures or levels of cloud coverage orsunshine, or surface conditions or textures of an environment betweenand including the origin and the destination.

In some embodiments, an aerial vehicle may be outfitted with a first setof one or more balanced propellers, as well as a second set of one ormore specific imbalanced propellers, or propellers that have varied ormodified parameters which cause the propellers to radiate sounds atspecific sound pressure levels or intensities and within specificfrequency spectrums during operation. During operation, power may beselectively applied to each of motors for rotating the propellers of thefirst set, or each of motors for rotating the propellers of the secondset, as necessary, in order to shape the sounds that are radiated fromthe aerial vehicle. A set of one or more imbalanced propellers may beselected based at least in part on one or more acoustic considerations,e.g., to emit sound at a predetermined sound pressure level (orintensity) and/or within a predetermined frequency spectrum. Forexample, where controlling or limiting the sounds emitted by an aerialvehicle is not a primary concern, the complement of propellers may beoperated, as necessary, in order to achieve any operational objectives(e.g., course, speed, payload, or the like). Where a specific sound isdesired to be emitted by the aerial vehicle, e.g., where the aerialvehicle operates within earshot of any humans or other animals, thepropellers of the complement may be operated, as necessary, in order toshape the overall sound profile of the aerial vehicle, and to cause thespecific sound to be emitted thereby.

Referring to FIGS. 1A through 1C, an aerial vehicle 110 and a pluralityof propellers 120A, 120B, 120C, 120D, 120E is shown. The aerial vehicle110 includes four motors 160A, 160B, 160C, 160D, each configured toreceive one of the propellers 120A, 120B, 120C, 120D, 120E. For example,each of the motors 160A, 160B, 160C, 160D may have a shaft configuredwith one or more bolted, quick-release, or other connections forinterchangeably mounting one of the propellers 120A, 120B, 120C, 120D,120E thereon.

As is shown in FIG. 1A, each of the propellers 120A, 120B, 120C, 120D,120E has different attributes. For example, the propeller 120A is abalanced propeller having two blades that is optimized for maximumthrust, and emits sound at a sound pressure level or intensity ofninety-five decibels (95 dB) and at a frequency of eight hundred fiftyhertz (850 Hz) during operations at a predetermined speed (e.g., angularvelocity). The propeller 120B is a balanced propeller having four bladesthat is configured for ultra-quiet operations, and emits sound at asound pressure level or intensity of seventy-four decibels (74 dB) andat a frequency of eight hundred ninety-eight hertz (898 Hz) duringoperations at a predetermined speed. The propeller 120C is an imbalancedpropeller having two blades, including a teardrop-shaped hole in one ofthe blades, that is configured for average lift and thrust, and emitssound at a sound pressure level or intensity of ninety-six decibels (96dB) and at a frequency of two thousand four hertz (2004 Hz) duringoperations at a predetermined speed.

The propeller 120D is a balanced propeller having three blades withangled tips that is configured for high maneuverability, and emits soundat a sound pressure level or intensity of eighty-four decibels (84 dB)and at a frequency of seven hundred eighty-seven hertz (787 Hz) duringoperations at a predetermined speed. The propeller 120E is an imbalancedpropeller having four blades, one of which is oversized or misshapen andincludes a round hole therein, that is configured for high lift, andemits sound at a sound pressure level or intensity of eighty decibels(80 dB) and at a frequency of three thousand eleven hertz (3011 Hz)during operations at a predetermined speed.

In accordance with the present disclosure, the aerial vehicle 110 may beoutfitted with one or more of each of the propellers 120A, 120B, 120C,120D, 120E, e.g., with sets having one or more of such propellers, whichmay be selected on any basis, e.g., to provide the aerial vehicle 110with specific thrust, lift or maneuverability capacities, or to causethe aerial vehicle 110 to emit a predetermined sound, and operatedseparately or in tandem and subject to any specific criteria,considerations or parameters such as their respective weights, shapes,or lift or drag profiles, as well as their angular velocities duringoperation. As is shown in FIG. 1B, the motors 160B, 160C are equippedwith the propellers 120A, and the motors 160A, 160D are equipped withthe propellers 160E. The propellers 120A and the propellers 120E arealigned to rotate within a common plane, or in parallel planes. As isfurther shown in FIG. 1B, the motors 160B, 160C are rotating thepropellers 120A under power as the aerial vehicle 110 travels in aspecific direction, and the motors 160A, 160D are not operating. Thepowered rotation of the propellers 120A by the motors 160B, 160C causesthe propellers 120A to emit sounds at a sound pressure level orintensity of ninety-five decibels (95 dB) and at a frequency of eighthundred fifty hertz (850 Hz) when the propellers 120A rotate at apredetermined speed (e.g., angular velocity).

As is discussed above, and in accordance with the present disclosure,the aerial vehicle 110 may operate one or more discrete sets ofpropellers, e.g., the propellers 120A and/or the propellers 120E, on anybasis. For example, when maximum thrust is required, the propellers 120Amay be operated. When maximum lift is required, the propellers 120E maybe operated. When both lift and thrust are considerations, thepropellers 120A, 120E may be operated concurrently, as necessary, atvarying power levels. Likewise, the propellers 120A, 120E may also beoperated separately where the respective sounds emitted by suchpropellers 120A, 120E during operation are desired, or concurrently, asnecessary, and at varying power levels, where a blend of such sounds isdesired, or where the sounds emitted by the aerial vehicle 110 are notessential.

As is shown in FIG. 1C, the motors 160A, 160D are rotating thepropellers 120E under power as the aerial vehicle 110 travels in aspecific direction, and the motors 160B, 160C are not rotating. Thepowered rotation of the propellers 120E by the motors 160A, 160D causesthe propellers 120E to emit sound pressure level or intensity of eightydecibels (80 dB) and at a frequency of three thousand eleven hertz (3011Hz) during operations at a predetermined speed.

Accordingly, the systems and methods of the present disclosure mayoperate different sets of propellers, including a set including one ormore intentionally imbalanced propellers, as may be required based onany specific criteria, considerations or parameters, e.g., in order toeffectively control or shape the noises emitted by such propellersduring operation of the aerial vehicle. An aerial vehicle includingmultiple propellers, or sets of propellers, may operate such propellersin an alternating fashion, such as the aerial vehicle 110 shown in FIGS.1B and 1C, in which one or more propellers is rotating under power whileone or more other propellers are not rotated under power. Alternatively,an aerial vehicle may control or throttle the power provided to two ormore sets of such propellers on any basis, e.g., to ensure that aspecific sound (e.g., a sound having a selected sound pressure level orintensity and within a selected frequency spectrum) is emitted by therotating propellers, including but not limited to any relevantoperational characteristics, environmental conditions or other factors.

In this regard, by outfitting an aerial vehicle with a complement ofpropellers including one or more propellers that are balanced (e.g., instatic balance and in dynamic balance), and one or more selectedpropellers that are imbalanced (e.g., out of static balance or dynamicbalance) to a predetermined degree or extent, the aerial vehicle may beconfigured to emit any of a variety of specific sounds, as desired,during operation by manipulating the power applied to motors to whichsuch propellers are mounted. An aerial vehicle so outfitted may,therefore, take the effects of noise on any humans or other animals intoconsideration as an operational constraint, and may instead beconfigured to emit pleasant or agreeable sounds when the aerial vehicleis within earshot of such humans or animals by rotating one or moreimbalanced propellers during operation. Alternatively, the aerialvehicle may be configured to emit a specific sound that may be neitherpleasant nor agreeable (e.g., a warning sound, a siren or an alarm) atpredetermined times or when one or more specific operationalcharacteristics or environmental conditions are observed.

An aerial vehicle may be outfitted with one or more propellers having avariety of different attributes that may be selected on any basis. Forexample, a propeller may be selected based on one or more parameterssuch as mass or one or more dimensions (e.g., radius or diameter, bladelength, blade width, blade shape or blade angle). Additionally, apropeller may be selected based on one or more operational capacities,e.g., a measure or rating of the thrust capacity, the lift capacity orthe speed capacity that may be provided by the propeller, a measure orrating of the maneuverability of an aerial vehicle equipped with such apropeller, or a measure or rating of the one or more sounds that may beemitted by the propeller during operation. A propeller may be selectedbased on a general level or degree of performance, or on a level ordegree of performance in specific instances, e.g., with regard tospecific goals or objectives such as maneuverability, fuel efficiencyand/or battery life, or adverse weather conditions.

Operating aerial vehicles may emit a number of different sounds atvarying sound pressure levels and within various frequency spectrumsduring operation. In many instances, sounds radiated from operatingaerial vehicles include broadband sounds, e.g., energies that aredistributed across wide bands or ranges of frequency, and narrowbandsounds or tonals, e.g., which are typically centered around discretefrequencies or narrower bands or ranges, and are commonly periodic orharmonic in nature. For example, where an aerial vehicle includes aplurality of propellers, sounds radiated by the propellers may bedetermined as functions of the blade pass frequencies (or blade passingfrequencies) of each of the propellers. A blade pass frequency is, asits name suggests, a frequency with which a blade on a rotatingpropeller passes a given point, and is determined as a function of anangular velocity of the rotating propeller and the number of bladesprovided on the rotating propeller. An operating aerial vehicletypically emits not only broadband sounds but also narrowband sounds,including strong fundamental tonal sounds at or near the blade passfrequency (or within a band that includes the blade pass frequency), anddiscrete sound elements at various harmonics of the blade passfrequency.

With regard to a rotating machine, the term “balance” (or “balanced”) isused to refer to a condition in which all forces generated by, or actingupon, a rotating element are in a state of equilibrium. Disruptions tothe state of equilibrium result in an “imbalance,” or an imbalancedcondition. A propeller (e.g., an aircraft or marine propeller) that isin static balance has a center of mass or gravity aligned along an axisof rotation. In such a condition, the propeller may spin about the axisof rotation without any net inertial forces acting thereon, such thatthe centrifugal forces associated with each of the blades of thepropeller are balanced about the axis of rotation accordingly.Propellers having centers of mass or gravity that are displaced from theaxis of rotation are said to be statically imbalanced, or out of staticbalance, and will generate net inertial forces during rotation. Apropeller that is in dynamic balance has a principal axis of inertiathat is not parallel to the axis of rotation, due to the fact that thecenters of gravity of the various blades of the propeller are not in thesame plane. In such a condition, rotation of the propeller will causethe propeller to vibrate or flutter at a critical speed that isdetermined based on an extent of the imbalance.

When a propeller is balanced, lower levels of structural, torsional, androtating-shaft vibrations are observed during operation. When apropeller is imbalanced, however, undesirable vibratory forces andexcessive noise levels are generated during operation. Traditionally,vibrations generated by the rotation of imbalanced propellers in aerialvehicles have been associated with increased risks of damage topropellers, shafts, bearings and other components. Accordingly, ownersand operators of propeller-driven aerial vehicles regularly act toensure that each of the propellers onboard such vehicles are properlybalanced, both statically and dynamically, in order to reduce not onlythe risk of damage but also the vibratory forces and noise emitted bysuch propellers during operation. Traditionally, each propeller mountedto or operated by an aerial vehicle is balanced, both statically anddynamically.

The systems and methods of the present disclosure are directed tooperating aerial vehicles with discrete sets of propellers that may beselected on any basis, e.g., operational criteria such as speed, lift,thrust, maneuverability, efficiency or noise, or any environmentalconditions anticipated during the performance of the mission. The aerialvehicle may then be operated with the complement of propellers operatingin any manner, e.g., alone or in any combination, in order to complete amission in accordance with a transit plan, in order to cause the aerialvehicle to emit any kind or type of sound, or for any other purpose. Byproviding an aerial vehicle with two or more discrete sets of suchpropellers, a single aerial vehicle may operate in two or more modesduring transit, with the respective sets of propellers operating aloneor in tandem with one or more other sets, e.g., at various angularvelocities and/or power levels that may be dynamically modulated asdesired or required based on the operational objectives or demands of aparticular mission. In some embodiments, each of the propellers in eachof the complements may rotate about vertical axes, e.g., with the bladesrotating in substantially horizontal planes that are common or inparallel to one another. In some other embodiments, the propellers ofthe various complements may rotate about different axes (e.g., at leastone of which may be non-vertical), and with the blades rotating indifferent planes (e.g., at least some of which may be non-horizontal).In still other embodiments, the axes of rotation of the propellersand/or the planes within which the blades of such propellers rotate maybe varied, e.g., by one or more motors or other components forrepositioning motors and/or propellers.

In some embodiments, the systems and methods disclosed herein aredirected to exploiting, not avoiding, the use of imbalanced propellersin aerial vehicles in order to cause such vehicles to emit specificsounds during operation. A set of propellers including one or moreimbalanced propellers may be provided on an aerial vehicle and operatedalone or in combination with one or more other propellers, e.g.,combinations of imbalanced propellers, or both balanced and imbalancedpropellers, and at angular velocities and/or power levels that cause theoverall sound profile of the aerial vehicle to vary accordingly.

Thus, in some embodiments, where an aerial vehicle is to be operatedwithin earshot of one or more humans or other animals, the aerialvehicle may be outfitted with a plurality of propellers including bothbalanced and imbalanced propellers that may be specifically selectedbased on various operational criteria, e.g., a thrust rating, a liftrating, a speed rating, a maneuverability rating or a noise rating ofthe respective propellers. Such propellers may then be operated, asnecessary, in order to achieve one or more operational objectives oraccording to one or more demands. In particular, when the aerial vehiclepasses within a vicinity of one or more humans or other animals, one ormore imbalanced propellers may be operated in order to cause the aerialvehicle to radiate a selected sound until the aerial vehicle safelydeparts from the vicinity of the humans or other animals. The sounds maybe selected based on the extent to which they are pleasant or annoyingto such humans or animals, or on any other basis.

In some embodiments, a balanced propeller may be rotated to apredetermined speed, e.g., to above a critical speed for the propeller.For example, the propeller may be rotated during the operation of anaerial vehicle, or in a laboratory or testing facility. One or moresensors, e.g., microphones, piezoelectric sensors, vibration sensors, orany other acoustic sensors, may be used to capture information regardingthe rotation of the propeller and any observed sounds or noises radiatedtherefrom. The observed sounds or noises may then be compared to soundsor noises that are desired to be emitted by the propeller duringoperation. If the observed sounds or noises are inconsistent with thedesired sounds or noises, then one or more modifications or adjustmentsmay be made to the propeller, e.g., to intentionally imbalance thepropeller, and the modified propeller may be rotated again. Suchmodifications may include the addition or subtraction of mass from oneor more of the blades of the propeller, or the modification of one ormore parameters of the propeller. If the sounds or noises observed fromthe rotation of the modified propeller are consistent with the desiredsounds or noises, then the propeller may be utilized during theoperation of an aerial vehicle accordingly. If the sounds or noisesobserved from the rotation of the modified propeller are inconsistentwith the desired sounds or noises, however, the propeller may be furthermodified with one or more modifications or adjustments and rotated againuntil such sounds or noises are sufficiently consistent with the desirednoises.

In accordance with some other embodiments of the present disclosure, apropeller complement may be selected for an aerial vehicle that isscheduled to perform a specific mission. For example, the complement ofpropellers may be selected in accordance with a transit plan (e.g., aroute from an origin to a destination, through any interveningwaypoints) for the mission, or any predicted operational characteristicsor environmental conditions anticipated during the performance of themission, e.g., based on one or more attributes of the mission, theroute, the origin, the destination, the waypoints, the operationalcharacteristics or the environmental conditions. For example, thecomplement of propellers may be selected by estimating the soundpressure levels or intensities and frequency spectrums of sounds to beemitted by each of a plurality of propellers during operation, andidentifying one or more desired sounds to be emitted by the aerialvehicle during the mission, and outfitting the aerial vehicle withpropellers that will cause the aerial vehicle to emit sounds that aresufficiently similar to the desired sounds. Once the complement ofpropellers has been selected and installed on the aerial vehicle, thepropellers may be operated, as necessary, in accordance with the transitplan or on any other basis.

In accordance with still other embodiments of the present disclosure, anaerial vehicle having a plurality of different onboard propellers maydepart from an origin for a destination along a route in an originaltransit mode for a mission, e.g., in which each of the propellers isoperating at a selected speed or at a particular power level, or is notoperating. One or more sensors operating onboard the aerial vehicle maytrack the position and/or altitude of the aerial vehicle, and capturedata regarding operational characteristics of the aerial vehicle orenvironmental conditions encountered by the aerial vehicle while in theoriginal transit mode. Such sensors may also determine the soundpressure levels and/or frequency spectrums of sounds emitted by theaerial vehicle while in transit. If the emitted sound pressure levelsand/or frequency spectrums are not consistent with a desired sound,e.g., a sound pressure level and/or a frequency spectrum that ispreferred, then the transit mode of the aerial vehicle may be changedaccordingly, e.g., based on one or more attributes of the mission, theroute, the origin, the destination, one or more waypoints, theoperational characteristics or the environmental conditions.

For example, where an aerial vehicle includes a first set of propellersfor operating in a first transit mode, and the aerial vehicle arrives ata predetermined location, reaches a predetermined speed, altitude orbattery level, or encounters a predetermined temperature or weatherevent, the aerial vehicle may be operated in a second transit mode inwhich a second set of propellers is operated either independently or intandem with one or more of the first set of propellers, and at powerlevels that may be desired based on one or more thrust, lift, efficiencyor acoustic considerations, or for any other purpose. In this regard, byequipping an aerial vehicle with two or more discrete sets ofpropellers, the aerial vehicle may be operated in a variety of differentmodes, and may be utilized to satisfy one or more operational objectivesat different times, or respond to changing events or circumstances whileperforming a mission.

As is discussed above, an aerial vehicle may be operated with any numberof discrete sets of propellers, and that such propellers may be eitherbalanced (e.g., in static balance or in dynamic balance) or imbalanced,as needed, in order to cause an aerial vehicle to emit sound having adesired sound pressure level or within a desired frequency spectrum. Inaccordance with the present disclosure, a propeller may be intentionallyimbalanced by effecting a change to one or more parameters of thepropeller, e.g., a change to a mass or weight, a shape, a lift profileor a drag profile of one or more blades of the propeller, which may beaccomplished in any number of ways. Referring to FIGS. 2A through 2I, aplurality of imbalanced propellers 220A, 220B, 220C, 220D, 220E, 220F,220G, 220H, 220I are shown. As is shown in FIG. 2A, the propeller 220Aincludes a pair of blades 230A, 240A mounted about a hub 250A having amounting bore 252A. The blade 240A has a length l_(2A) that is greaterthan a length l_(1A) of the blade 230A. Accordingly, due to theirdifferent lengths l_(2A), l_(1A), the blades 230A, 240A have differentcenters of mass with respect to the hub 250A, and the propeller 220Awill be imbalanced (i.e., out of static balance or dynamic balance) whenoutfitted to an aerial vehicle (not shown) and rotated under power. Theimbalance of the propeller 220A will cause the propeller 220A to radiatesounds at a discrete sound pressure level or intensity and within afinite frequency spectrum that may be different from the sounds radiatedfrom a balanced propeller that is similarly sized.

As is shown in FIG. 2B, the propeller 220B includes a pair of blades230B, 240B mounted about a hub 250B having a mounting bore 252B. Theblade 240B has a width w_(2B) that is greater than a width w_(1B) of theblade 230B. Accordingly, due to their different widths w_(2B), w_(1B),the blades 230B, 240B have different centers of mass with respect to thehub 250B, and the propeller 220B will also be imbalanced when outfittedto an aerial vehicle (not shown) and rotated under power. The imbalanceof the propeller 220B will cause the propeller 220B to radiate sounds ata discrete sound pressure level or intensity and within a finitefrequency spectrum that may be different from the sounds radiated from abalanced propeller that is similarly sized. Likewise, as is shown inFIG. 2C, the propeller 220C includes a pair of blades 230C, 240C mountedabout a hub 250C having a mounting bore 252C. The blade 230C includes asubstantially circular hole 232C having an area A_(1C) provided at adistance l_(1C) from a center of the mounting bore 252C. The blade 240Cincludes a substantially ellipsoidal hole 242C having an area A_(2C)provided at a distance l_(2C) from a center of the mounting bore 252C.Accordingly, due to their different distributions of mass resulting fromthe holes 232C, 242C and the placements thereof with respect to themounting bore 252C, the propeller 220C will also be imbalanced whenoutfitted to an aerial vehicle (not shown) and rotated under power. Theimbalance of the propeller 220C will cause the propeller 220C to radiatesounds at a discrete sound pressure level or intensity and within afinite frequency spectrum that may be different from the sounds radiatedfrom a balanced propeller that is similarly sized.

As is shown in FIG. 2D, the propeller 220D includes a pair of blades230D, 240D mounted about a hub 250D having a mounting bore 252D. Theblade 230D has an opening 232D having an area A_(1D), and an adjustablecover 234D for concealing or exposing the opening 232D. The blade 240Dhas an opening 242D having an area A_(2D), and an adjustable cover 244Dfor concealing or exposing the opening 242D. The adjustable covers 234D,244D may be operated to conceal or expose the respective openings 232D,242D in any manner, e.g., radially, laterally or in any directionrelative to the openings 232D, 242D, using one or more electrical motorsor other systems. Accordingly, depending on whether one or more of theadjustable covers 234D, 244D is concealing or exposing one or more ofthe openings 232D, 242D, the propeller 220D may be either balanced orautomatically and selectively imbalanced, as needed, during operations.The adjustable covers 234D, 244D of the propeller 220D may therefore beused to determine the sound pressure level or intensity and frequencyspectrum of the sounds radiated from the propeller 220D duringoperations.

As is shown in FIG. 2E, the propeller 220E includes a pair of blades230E, 240E mounted about a hub 250E having a mounting bore 252E. Theblade 230E has an opening 232E into which a slug 234E is inserted. Theopening 232E is provided at a distance l_(1E) from a center of themounting bore 252E. The blade 240E has an opening 242E from which a core244E was removed. The opening 242E is provided at a distance l_(2E) fromthe center of the mounting bore 252E. Accordingly, depending on the massor density of the slug 234E, or the distances l_(1E), l_(2E) from theopenings 232E, 242E to the center of the mounting bore 252E, the blades230E, 240E have different centers of mass with respect to the hub 250E,and the propeller 220E will be imbalanced when outfitted to an aerialvehicle (not shown) and rotated under power. The imbalance of thepropeller 220E will cause the propeller 220E to radiate sounds at adiscrete sound pressure level or intensity and within a finite frequencyspectrum that may be different from the sounds radiated from a balancedpropeller that is similarly sized.

As is shown in FIG. 2F, the propeller 220F includes a pair of blades230F, 240F mounted about a hub 250F having a mounting bore 252F. Theblades 230F, 240F are formed from different materials m_(1F), m_(2F)having different densities ρ_(1F), ρ_(2F). Accordingly, the blades 230F,240F will have different centers of mass with respect to the hub 250F,even if the blades 230F, 240F have identical dimensions, and thepropeller will be imbalanced when outfitted to an aerial vehicle (notshown) and rotated under power. The imbalance of the propeller 220F willcause the propeller 220F to radiate sounds at a discrete sound pressurelevel or intensity and within a finite frequency spectrum that may bedifferent from the sounds radiated from a balanced propeller that issimilarly sized, with the sound pressure level or intensity andfrequency spectrum being determined based on s difference between therespective densities ρ_(1F), ρ_(2F).

As is shown in FIG. 2G, the propeller 220G includes a pair of blades230G, 240G mounted about a hub 250G having a mounting bore 252G. Theblades 230G, 240G are provided at different rake angles θ_(1G), θ_(2G)with respect to the hub 250G. Accordingly, the blades 230G, 240G willhave different centers of mass with respect to the hub 250G, even if theblades 230G, 240G have identical dimensions, and the propeller will beimbalanced when outfitted to an aerial vehicle (not shown) and rotatedunder power. The imbalance of the propeller 220G will cause thepropeller 220G to radiate sounds at a discrete sound pressure level orintensity and within a finite frequency spectrum that may be differentfrom the sounds radiated from a balanced propeller that is similarlysized, with the sound pressure level or intensity and frequency spectrumbeing determined based on s difference between the respective bladeangles θ_(1G), θ_(2G).

As is shown in FIG. 2H, the propeller 220H includes a pair of blades230H, 240H mounted about a hub 250G having a mounting bore 252H. Theblades 230H, 240H are provided with different thicknesses t_(1H), t_(2H)of their respective leading and trailing edges. Accordingly, the blades230G, 240G will have different centers of mass with respect to the hub250G, even if the blades 230G, 240G have identical dimensions, and thepropeller will be imbalanced when outfitted to an aerial vehicle (notshown) and rotated under power. The imbalance of the propeller 220G willcause the propeller 220G to radiate sounds at a discrete sound pressurelevel or intensity and within a finite frequency spectrum that may bedifferent from the sounds radiated from a balanced propeller that issimilarly sized, with the sound pressure level or intensity andfrequency spectrum being determined based on s difference between therespective blade angles θ_(1G), θ_(2G).

As is shown in FIG. 2I, the propeller 220I includes a pair of blades230I, 240I mounted about a hub 250I having a mounting bore 252I. Theblades 230I, 240I are provided at different pitch angles φ_(1I), φ_(2I)with respect to the hub 250I. Accordingly, the blades 230I, 240I willhave different centers of mass with respect to the hub 250I, even if theblades 230I, 240I have identical dimensions, and the propeller will beimbalanced when outfitted to an aerial vehicle (not shown) and rotatedunder power. The imbalance of the propeller 220I will cause thepropeller 220I to radiate sounds at a discrete sound pressure level orintensity and within a finite frequency spectrum that may be differentfrom the sounds radiated from a balanced propeller that is similarlysized, with the sound pressure level or intensity and frequency spectrumbeing determined based on s difference between the respective rakeangles φ_(1I), φ_(2I).

The aerial vehicles and propellers disclosed herein are not directed toany specific type or form of balanced or imbalanced propeller, or anyspecific process or technique for imbalancing a propeller, and are notlimited to any of the propellers 220A, 220B, 220C, 220D, 220E, 220F,220G, 220H, 220I of FIGS. 2A through 2I. For example, any number or typeof propellers, including but not limited to one or more of thepropellers disclosed in U.S. patent application Ser. No. 14/975,209,filed Dec. 18, 2015, the contents of which are incorporated by referenceherein in their entirety, may be operated on an aerial vehicle inaccordance with the systems and methods of the present disclosure.

According to some embodiments of the present disclosure, a propeller maybe modified or customized in any manner, e.g., by modifying one or moreparameters of one or more of the propeller, such as a mass, a shape, alift profile or a drag profile of one or more of the blades of thepropeller, in order to cause the propeller to radiate sounds atpredetermined or desirable sound pressure levels or intensities, andwithin selected frequency spectrums. The extent to which a propellermust be modified may be determined through one or more operational orexperimental analyses, which may be based upon characteristics of soundsradiated from the propeller during operation as compared tocharacteristics of desired sounds in order to identify or determine amodification to the propeller that may cause the propeller to radiatethe desired sounds, or sufficiently similar sounds, during operation.

Referring to FIG. 3, flow chart 300 of one process for operating apropeller in accordance with embodiments of the present disclosure isshown. At box 310, a balanced propeller is rotated to above a criticalspeed. As is noted above, a critical speed is an angular velocity of apropeller at which resonance, or vibration or oscillation of thepropeller at a specific frequency (e.g., a resonant frequency) or withina frequency spectrum, begins to occur. At box 320, one or more sensorsmay be used to capture information regarding the rotation of thepropeller and any observed noise emitted or radiated from the propeller.For example, where the balanced propeller is rotated to above thecritical speed on an aerial vehicle in flight, the sensors may beprovided about the shaft by which the propeller is mounted to a motor,or along a frame, a wing, a fuselage or one or more aspects of theaerial vehicle. Where the balanced propeller is rotated to above thecritical speed in an experimental environment (e.g., a laboratory orlike testing facility), the sensors may be positioned in anyadvantageous or desired location with respect to the rotating propeller.

At box 330, characteristics of the noise observed at box 320 arecompared to characteristics of a desired noise to be emitted or radiatedby the propeller during rotation above the critical speed. For example,where it is preferred or desired that a propeller emit sounds at aspecific sound pressure level or intensity and within a specificfrequency spectrum (e.g., a sound that may be more pleasant or soothingthan a sound ordinarily emitted from an operating balanced propeller, ora sound that may be obnoxious, annoying or louder than a soundordinarily emitted from an operating balanced propeller, and intended toalert or vex any nearby humans or other animals), a difference betweenthe sound pressure levels or intensities and frequency spectrums of thenoise observed at box 320 may be compared to the specific sound pressurelevel or intensity and frequency spectrum of the desired noise.

At box 340, whether the noise observed at box 320 is consistent with thedesired noise is determined. For example, one or more frequencies of theobserved noise may be compared to one or more frequencies of the desirednoise, e.g., to one or more desired frequency spectrums. Likewise, thesound pressure levels or intensities of the observed noise may besimilarly compared to the sound pressure levels or intensities of thedesired noise, or to one or more bands or tolerances associatedtherewith. If the observed noise is determined to be consistent with thedesired noise to within an acceptable level or degree, then the processends.

If the observed noise is determined to be inconsistent with the desirednoise, then the process advances to box 350, where any differencesbetween the observed noise and the desired noise are determined. Forexample, differences between the sound pressure levels or intensities ofthe observed noise and the desired noise may be assessed, e.g., bothindependently and with respect to one or more operating characteristicsof the aerial vehicle, including but not limited to the angular velocityat which the propeller is rotating. At box 360, one or more adjustmentsto the blades of the propeller may be identified to address thedifferences between the observed noise and the desired noise identifiedat box 350. For example, a mismatch between the masses, or the centersof mass, of two or more of the blades of the propeller intended toreduce or eliminate such differences may be proposed or selected.

At box 370, the propeller is modified in accordance with the determinedadjustments. For example, a length or a width of one of the blades ofthe balanced propeller may be reduced, e.g., by one or more cutting,bending or slicing means, thereby resulting in an imbalanced propellersuch as the propeller 220A of FIG. 2A or the propeller 220B of FIG. 2B.Alternatively, one or more holes may be formed within one or more of theblades, thereby resulting in an imbalanced propeller such as thepropeller 220C of FIG. 2C. One or more adjustable covers may be operatedto expose or conceal an opening in one or more of the blades, therebyresulting in an imbalanced propeller such as the propeller 220D of FIG.2D. A slug may be inserted into a blade, or a core may be removed from ablade, thereby resulting in an imbalanced propeller such as thepropeller 220E of FIG. 2E. Any means or method for changing the centersof mass of one or more blades of a propeller, or the locations of suchcenters, e.g., cutting, carving, sanding, rubbing, wearing, drilling,boring, or the like, may be utilized in order to modify a propellerbased on the one or more adjustments identified at box 360. Once thepropeller has been modified in accordance with the determinedadjustments, the process advances to box 380, where the modifiedpropeller is rotated to above a critical speed, and returns to box 320,where one or more sensors may capture information regarding the rotationof the propeller and any observed noise emitted or radiated from thepropeller.

The evaluation of a propeller that is in static balance and in dynamicbalance, and the modification of the balanced propeller to imbalance thepropeller in an effort to cause the propeller to emit sound at apreferred sound pressure level or intensity and within a preferredfrequency spectrum is shown in FIGS. 4A and 4B, in which views ofaspects of a balanced propeller 420A and an imbalanced propeller 420B inaccordance with embodiments of the present disclosure are shown. Exceptwhere otherwise noted, reference numerals preceded by the number “4”shown in FIG. 4A or FIG. 4B indicate components or features that aresimilar to components or features having reference numerals preceded bythe number “2” shown in FIGS. 2A through 2I or by the number “1” shownin FIGS. 1A and 1B.

As is shown in FIG. 4A, the balanced propeller 420A may be rotated at orabove a critical speed, and the rotation of the balanced propeller 420Amay be monitored, e.g., to determine one or more characteristics ofnoise observed during the rotation (e.g., a sound pressure level orintensity). Information or data regarding such noises may be stored inat least one data store, along with characteristics of the propeller420A (e.g., a mass, a diameter, a number of blades of the propeller420A, as well as an angular velocity at which the propeller 420A isrotated).

As is discussed above with regard to the flow chart 300 of FIG. 3, thecharacteristics of the noise observed during the rotation of thepropeller 420A may be compared to characteristics of a desired noise tobe emitted by the propeller 420A during operation, and one or moremodifications to the propeller 420A may be selected or predicted forresolving the differences between such characteristics. Referring toFIG. 4B, the propeller 420B is shown as being modified not only toinclude a slug 434B into a first blade 430B at a distance of l₂ from acenter of the propeller 420B, but also to remove a core 444B into asecond blade 440B at a distance of l₁ from the center of the propeller420B. After the slug 434B has been inserted into the first blade 430B,and the core 444B has been removed from the second blade 440B, thepropeller 420B may be rotated at or above a critical speed, and therotation of the balanced propeller 420B may be monitored to determineone or more characteristics of noise observed during the rotation (e.g.,a sound pressure level or intensity). Information or data regarding suchnoises may be stored in at least one data store, along withcharacteristics of the propeller 420B (e.g., a mass, a diameter, anumber of blades of the propeller 420B, as well as an angular velocityat which the propeller 420B is rotated, and one or more dimensions ofthe slug 434B or the core 444B). The information or data regarding thenoises and the propellers 420A, 420B may be used to develop and maintaina library of information or data regarding propellers of various sizes,shapes or configurations and the noises emitted thereby during operationwhen such propellers are in static balance and/or in dynamic balance, orwhen such propellers are in varying degrees of imbalance.

As is also discussed above, an aerial vehicle may be outfitted with acomplement of propellers, each of which may be selected on any basis.For example, in some embodiments, an octo-copter may be equipped withsets of propellers that are configured for optimized performance basedon different criteria. The octo-copter may be equipped with a first setof propellers generally configured for maximizing lift or thrust, and asecond set of propellers optimized for specific goals or objectives suchas maneuverability, fuel efficiency and/or battery life, or adverseweather conditions. Each of the sets of propellers may include as few asone and as many as seven propellers. During operation, the first set andthe second set of propellers may be selectively operated, as necessary,either individually or collectively, depending on the demands orrequirements set forth in a transit plan for a mission to be performedusing the aerial vehicle. Such demands or requirements may include, butare not limited to, limits or thresholds on acoustic emissions from theaerial vehicle in transit.

Referring to FIG. 5, a flow chart 500 of one process for operating anaerial vehicle in accordance with embodiments of the present disclosureis shown. At box 510, a transit plan is identified for a transit of anaerial vehicle from an origin to a destination. The transit plan mayinclude or indicate coordinates of the origin and/or the destination, aswell as times when the aerial vehicle is expected to depart from theorigin or arrive at the destination, or any other information or dataregarding the transit of the aerial vehicle. The transit plan mayfurther include a preferred route to be traveled by the aerial vehiclefrom the origin to the destination (e.g., courses, speeds, orcoordinates of one or more waypoints), any altitude restrictions orrequirements, or any information or data regarding a payload to becarried by the aerial vehicle from the origin to the destination.

At box 520, operational characteristics of the aerial vehicle during thetransit in accordance with the transit plan are predicted. For example,any dynamic attributes such as altitudes, courses, speeds, rates ofclimb or descent, turn rates, accelerations or tracked positions (e.g.,latitudes and/or longitudes) of the aerial vehicle during the transitmay be estimated based on the transit plan and any known relevantfactors or historical data. At box 530, any environmental conditions tobe encountered by the aerial vehicle during the transit are alsopredicted. For example, any temperatures, pressures, humidities, windspeeds, directions, measures of cloud coverage, sunshine, or surfaceconditions or textures of an environment between and including theorigin and the destination may also be estimated based on the transitplan and any known relevant factors or historical data.

At box 540, sound pressure levels and/or frequency spectrums of soundsthat will preferably be emitted by the aerial vehicle are determinedbased on the transit plan, the predicted operational characteristics,and/or the predicted environmental conditions. For example, based on anyrequirements or constraints associated with or imposed by the transitplan, the anticipated operational characteristics of the aerial vehicleor the environmental conditions that the aerial vehicle is expected toencounter, a preferred sound to be emitted by the aerial vehicle (e.g.,sound pressure levels and frequency spectrums) may be identified. Suchcharacteristics may define or include a single threshold or limit (e.g.,a minimum value, a maximum value or an average value of a sound pressurelevel or frequency), or one or more thresholds or limits (e.g., aspectrum, a distribution, a pattern or a band) depending on a velocity,a position or an altitude of the aerial vehicle, or any other factor.For example, a first sound pressure level and/or first frequencyspectrum of sounds to be emitted within a vicinity of the origin (e.g.,within earshot of humans or other animals) may be determined, while asecond sound pressure level and/or frequency spectrums of sounds to beemitted while the aerial vehicle is in transit, and a third soundpressure level and/or frequency spectrums of sounds to be emitted whilethe aerial vehicle is within a vicinity of the destination may also bedetermined. Sound pressure levels and/or frequency spectrums of soundsto be emitted within a vicinity of any number of intervening waypointsmay also be determined. Any number of sound pressure levels and/orfrequency spectrums to be emitted by the aerial vehicle, e.g., at anyintervals or for any durations, may be identified based on the transitplan, the predicted operational characteristics, and/or the predictedenvironmental conditions.

At box 550, a propeller complement for an aerial vehicle is selectedbased on the sound pressure levels and/or frequency spectrums of thepreferred sounds. The aerial vehicle may be outfitted with a homogenousarray of propellers based on the desired sound pressure levels and/orfrequency spectrums, or with two or more sets of propellers, that areconfigured to cause the aerial vehicle to emit sounds that areconsistent with the desired sound pressure levels and/or frequencyspectrums. For example, in some embodiments, one or more of thepropellers may be intentionally imbalanced to a predetermined extent inorder to cause the aerial vehicle to emit a specific sound (e.g., soundat a specific sound pressure level and/or within a specific frequencyspectrum) when such propellers are operating. Alternatively, those ofordinary skill in the pertinent arts will recognize that the propellercomplement may be selected based on factors or criteria that areunrelated to the desired sound pressure levels and/or frequencyspectrums in accordance with the present disclosure.

The aerial vehicles of the present disclosure may include any number ofsets of propellers, and such sets may include any number of propellers.For example, a quad-copter may include a single set of four commonpropellers selected specifically for the purpose of emitting sound at adesired sound pressure level and/or within a desired frequency spectrum,or two or more sets of propellers selected for any specific purpose(e.g., two propellers for optimal thrust, two propellers for emittingsound at the desired sound pressure level and/or within a desiredfrequency spectrum; or four unique propellers, including one propellerfor optimal battery life, one propeller for optimal performance inadverse weather conditions, one propeller for optimal lift and onepropeller for emitting sound at the desired sound pressure level and/orwithin a desired frequency spectrum). Likewise, as other examples, asix-propeller aerial vehicle (e.g., a hexa-copter) may include a singleset of six common propellers, or two or more sets of propellers (e.g.,two sets of three propellers each; three sets of two propellers each;three sets of propellers including three propellers, two propellers anda single propeller each; or six unique propellers) that may be selectedfor any specific purpose, and an eight-propeller aerial vehicle (e.g.,an octo-copter) may include two to eight unique sets of propellers thatmay also be selected for any specific purpose.

At box 560, the aerial vehicle is outfitted with the selected propellercomplement. At box 570, the aerial vehicle departs from the origin forthe destination with the selected propeller complement installed, andthe process ends.

Accordingly, the systems and methods of the present disclosure may bedirected to identifying and selecting propellers to be installed on anaerial vehicle prior to departing on a predetermined mission inaccordance with a transit plan. In some embodiments, two or more sets ofpropellers may be selected for and mounted to the aerial vehicle priorto departure, and operated at specific times or at specific angularvelocities in accordance with the transit plan. The sets of propellersmay be selected for an aerial vehicle in order to cause the aerialvehicle to emit a predetermined sound during operation, such as is shownin the flow chart 500 of FIG. 5, and may include one or moreintentionally imbalanced propellers, as necessary. Those of ordinaryskill in the pertinent arts will recognize that sets of propellers maybe selected on any basis other than a desired sound to be emitted, e.g.,for lift, thrust, maneuverability or efficiency considerations at one ormore points or along one or more segments of a transit plan, and thatthe sets of propellers may be operated based on such considerations.

The selection and use of different complements of propellers inaccordance with different transit plans may be shown in FIG. 6.Referring to FIG. 6, views of aspects of one system 600 for operatingaerial vehicles in accordance with embodiments of the present disclosureare shown. Except where otherwise noted, reference numerals preceded bythe number “6” shown in FIG. 6 indicate components or features that aresimilar to components or features having reference numerals preceded bythe number “4” shown in FIG. 4A or FIG. 4B, by the number “2” shown inFIGS. 2A through 2I or by the number “1” shown in FIGS. 1A and 1B.

As is shown in FIG. 6, a plurality of aerial vehicles 610-1, 610-2, and610-3 is selected to execute one or more missions in accordance withtransit plans. The aerial vehicle 610-1 is intended to travel fromHartford, Conn., to Southport, Conn., while carrying a twenty-eightpound (28 lbs.) payload. In accordance with a transit plan, the aerialvehicle 610-1 is expected to travel at a course of 194 degrees (194°),along a route that may be generally characterized as covering orparalleling highway routes and passing over suburban communities.Therefore, the aerial vehicle 610-1 will be outfitted with twohigh-efficiency propellers and two low-noise propellers, which may beoperated separately or in tandem, and at different power levels, asnecessary.

For example, the high-efficiency propellers may be operated at fullpower for much of the fifty-three mile transit from Hartford toSouthport, e.g., portions of the transit within urban environments orpassing over highways. The low-noise propellers may be operated as theaerial vehicle 610-1 passes over or within a vicinity of humans or otheranimals, or dwellings including such humans or animals, e.g., near thedestination, in order to ensure that the sound emitted by the aerialvehicle 610-1 remains below a predetermined threshold. Alternatively,the sets of propellers may be operated together, at varying powerlevels, with the high-efficiency propellers being operated atcomparatively higher power levels where conservation is a priority, andwith the low-noise propellers being operated at comparatively higherpower levels where noise control is a priority. Moreover, in someembodiments, one or both of the low-noise propellers or thehigh-efficiency propellers may be intentionally imbalanced in order tocause the aerial vehicle to emit predetermined sounds when thepropellers are rotated under power during operation.

As is also shown in FIG. 6, the aerial vehicle 610-2 is intended totravel from Hartford to Groton, Conn., while carrying a three-pound (3lbs.) payload. In accordance with a transit plan, the aerial vehicle610-2 is expected to travel at a course of 132 degrees (132°), along asparsely populated route. Therefore, because the route is sparselypopulated, a noise threshold associated with the route is relativelyhigh, and the aerial vehicle 610-2 may be outfitted with four high-speedpropellers to enable the aerial vehicle 610-2 to deliver the payload toGroton and return to Hartford at a maximum speed, without regard to anynoise emitted during the transit. As is further shown in FIG. 6, theaerial vehicle 610-3 is intended to travel from Hartford to Storrs,Conn., while carrying a thirty-five pound (35 lbs.) payload. Inaccordance with a transit plan, the aerial vehicle 610-3 is expected totravel at a course of 082 degrees (082°), along a dense routeterminating at a campus. Given the substantially high mass of thepayload carried by the aerial vehicle 610-3, and operational constraintsassociated with landing the aerial vehicle 610-3 in a compactenvironment, the aerial vehicle 610-3 may be outfitted with twopropellers having maximum lift capacities, and two propellers forprecision control (e.g., high maneuverability). The maximum lift andprecision control propellers may be operated separately or in tandem,and at varying power levels, as necessary, during the performance of themission.

As is discussed above, where an aerial vehicle is outfitted with two ormore unique sets of propellers, at least one of which may include one ormore intentionally imbalanced propellers, the sets of propellers may beoperated, as necessary, in order to cause the aerial vehicle to emit oneor more predetermined sounds

Referring to FIG. 7, a flow chart 700 representing one process foroperating an aerial vehicle having an imbalanced propeller in accordancewith embodiments of the present disclosure is shown. At box 710, anaerial vehicle having a plurality of different onboard propellersdeparts from an origin for a destination with the onboard propellers inan original transit mode. For example, the aerial vehicle may beoutfitted with separate sets of propellers that are optimized fordifferent purposes or criteria, e.g., a set of propellers for maximizinglift or thrust, and a set of propellers optimized for maneuverability,power efficiency, prevailing weather conditions or emitting a specificnoise, which may include one or more balanced or imbalanced propellers.

At box 720, one or more onboard sensors track the position of the aerialvehicle. At box 730, one or more onboard sensors capture data regardingenvironmental conditions and/or operational characteristics of theaerial vehicle during the transit from the origin to the destination. Atbox 740, one or more onboard sensors determine the sound pressure levelsand/or frequency spectrums of sounds being emitted by the aerial vehicleduring the transit from the origin to the destination. For example, theaerial vehicle may include one or more GPS receivers or sensors,compasses, speedometers, altimeters, gyroscopes, or other sensors fordetermining the position as well as the velocity or acceleration of theaerial vehicle, or any other operational characteristics of the aerialvehicle, while the aerial vehicle is in flight. The aerial vehicle mayfurther include one or more air monitoring sensors (e.g., oxygen, ozone,hydrogen, carbon monoxide or carbon dioxide sensors), infrared sensors,ozone monitors, pH sensors, magnetic anomaly detectors, metal detectors,radiation sensors (e.g., Geiger counters, neutron detectors, alphadetectors), attitude indicators, depth gauges, accelerometers or imagingdevices (e.g., digital cameras). The aerial vehicle may also include oneor more sound sensors for detecting and capturing sound energy while theaerial vehicle is in flight, including one or more microphones,piezoelectric sensors, vibration sensors, or any other device configuredto capture information or data regarding acoustic energy.

At box 750, sound pressure levels and/or frequency spectrums of soundsthat will preferably be emitted by the aerial vehicle while the aerialvehicle is in flight are determined based on the tracked position, theenvironmental conditions and/or the operational characteristics. Forexample, in some embodiments, a preferred sound to be emitted by anaerial vehicle may be determined based on a position of the aerialvehicle (e.g., a first sound may be preferably emitted when the aerialvehicle is within earshot of humans or other animals, and a second soundmay be preferably emitted when the aerial vehicle is out of range ofsuch humans or animals, or when ambient noise levels are sufficientlyhigh and the operating sounds emitted by the aerial vehicles arecomparatively insignificant.

In some embodiments, e.g., when the aerial vehicle is expected tooperate near dwellings or other inhabited buildings, the preferredsounds to be emitted when the aerial vehicle is within range ofstructures may have low sound pressure levels and/or frequency spectrumsthat are known not to annoy humans or animals. In some otherembodiments, e.g., where it is desired to warn any humans or animals ofan arriving or departing aerial vehicle, the preferred sounds to beemitted when the aerial vehicle is within range of dwellings or otherinhabited buildings may have a high sound pressure level, e.g.,approximately one hundred decibels (dB), and/or a frequency spectrumthat is known to annoy such humans or animals, e.g., frequencies withina range of three thousand to four thousand Hertz (3000-4000 Hz).Moreover, in some other embodiments, the preferred sound may beidentified as a function of the velocity of the aerial vehicle, thealtitude of the aerial vehicle, a size (e.g., a net mass) of a payloadcarried by the aerial vehicle, weather conditions encountered by theaerial vehicle, or any other relevant environmental or operationalfactor.

In accordance with the present disclosure, desired sound pressure levelsand/or frequency spectrums may be determined based not only on existingregulatory, statutory or procedural requirements but also on historicaldata, e.g., by providing information or data regarding the position ofthe aerial vehicle, the operating characteristics of the aerial vehicle,or the environmental conditions within which the aerial vehicle isoperating as inputs to a machine learning system trained to recognizepreferred sounds. Desired sounds may also be identified based onoperational events such as passing above or below a predeterminedaltitude, exceeding or falling below a predetermined airspeed, orarriving within or departing from a range of a predetermined location.The information or data utilized to identify desired sounds may beweighted based on the reliability of extrinsic or intrinsic informationor data determined at box 720, box 730 or box 740 using onboard sensors(e.g., an extent to which the information or data may be expected toremain constant), the quality of the predicted extrinsic or intrinsicinformation or data (e.g., a level of confidence in estimates orforecasts on which such information or data is derived), or on any otherfactor.

At box 760, the sound pressure levels and/or frequency spectrums of thepreferred sounds to be emitted by the aerial vehicle as determined atbox 750 are compared to the sound pressure levels and/or frequencyspectrums of sounds being emitted by the aerial vehicle during operationas determined at box 740. For example, the extent to which the soundpressure levels and/or intensities of sounds being emitted by the aerialvehicle deviate from the desired sound pressure levels or intensitiesand/or desired frequency spectrums may be determined. At box 770, if thesound pressure levels and/or frequency spectrums of sounds being emittedby the aerial vehicle are not sufficiently different from those of thedesired sounds, then the process returns to box 720, where the aerialvehicle continues to operate in accordance with the original transitmode, and where the position of the aerial vehicle is tracked using oneor more onboard sensors.

If the sound pressure levels and/or frequency spectrums of the soundsemitted by the aerial vehicle are sufficiently different from those ofthe desired sounds, then the process advances to box 780, where atransit mode of the onboard propellers is changed. For example, wherethe aerial vehicle is outfitted with two or more sets of discretepropellers, and where a first set of the propellers is operating in theoriginal transit mode, a second set of the propellers may be operated ina subsequent transit mode, and the first set of the propellers may bestopped. Likewise, where the aerial vehicle is outfitted with a firstset of discrete propellers and a second set of discrete propellers, eachoperating at a first power level and a second power level, respectively,the power applied to each of the first set and the second set ofpropellers may be throttled or changed to vary the sounds emitted by theaerial vehicle during operation. In some embodiments of the presentdisclosure, one or more imbalanced propellers may be operated orstopped, as necessary, in order to modify the sounds emitted by theaerial vehicle during operations. In still other embodiments, the stateof balance of one or more of the propellers may be automaticallychanged, e.g., by exposing or concealing one or more openings providedwithin a blade of a propeller, such as the propeller 220D of FIG. 2D, orin any other manner. The operating status of any number of propellersprovided on an aerial vehicle may be modified in any manner, asnecessary, in order to alter the sound pressure levels and/or frequencyspectrums emitted by the aerial vehicle in accordance with the presentdisclosure.

At box 790, whether the aerial vehicle has arrived at its destination isdetermined. If the aerial vehicle has arrived at its destination, theprocess ends. If the aerial vehicle has not arrived at its destination,however, then the process returns to box 720, where the position of theaerial vehicle is tracked using one or more onboard sensors.

Accordingly, the systems and methods of the present disclosure may beutilized to modify a transit mode of an aerial vehicle, as necessary, orto operate an aerial vehicle in two or more transit modes, in order tochange a sound pressure level or intensity and/or a frequency spectrumof sounds emitted by the aerial vehicle. The transit mode may bemodified by starting or stopping the operation of one or morepropellers, e.g., one or more balanced or imbalanced propellers, or bymodifying the power applied to one or more of such propellers. Themodifications to the transit mode may be identified and implementedbased on the position of the aerial vehicle, or based on any operatingcharacteristics of the aerial vehicle (e.g., altitudes, courses, speeds,rates of climb or descent, turn rates, accelerations), or anyenvironmental conditions encountered by the aerial vehicle (e.g.,temperatures, pressures, humidities, wind speeds or directions, measuresof cloud coverage or sunshine, or surface conditions or textures) withina given environment.

One example in which a transit mode of an aerial vehicle may be changedin response to sounds being by the aerial vehicle, or in order to causepreferred or desired sounds to be emitted by the aerial vehicle, isshown in FIG. 8. Referring to FIG. 8, a view of aspects of an operatingaerial vehicle 810 in accordance with embodiments of the presentdisclosure is shown. Except where otherwise noted, reference numeralspreceded by the number “8” shown in FIG. 8 indicate components orfeatures that are similar to components or features having referencenumerals preceded by the number “6” shown in FIG. 6, by the number “4”shown in FIG. 4A or FIG. 4B, by the number “2” shown in FIGS. 2A through2I or by the number “1” shown in FIGS. 1A and 1B.

The aerial vehicle 810 is equipped with a propeller complement includinga first set of high-speed propellers (Set A) and a second set ofultra-quiet propellers (Set B) and is intended to travel from an originin Boston, Mass., to a destination in Chatham, Mass. The aerial vehicle810 is slated to depart from Boston with a 28.6 pound (28.6 lbs.)payload at 1 o'clock in the afternoon on Jun. 27, 2015, on a course of127 degrees (127°), for a seventy-four mile (74 mile) transit toChatham.

As is shown in FIG. 8, the route to be traveled by the aerial vehicle810 is broken into four segments. Initially, upon departing from Boston,the aerial vehicle 810 is expected to travel over a dense, urbanenvironment for approximately sixteen minutes, and a distance ofapproximately eleven miles. During this segment, the first set ofhigh-speed propellers may be operated at eighty percent (80%) power, andthe second set of ultra-quiet propellers may be operated at twentypercent (20%) power. Next, after the aerial vehicle 810 clears thedense, urban environment, the aerial vehicle 810 is expected to reach afirst intervening waypoint and enter a coastal protected zone, throughwhich the aerial vehicle will travel for approximately twenty-sixminutes and for a distance of approximately fifteen miles. Within thecoastal protected zone, the first set of high-speed propellers may beoperated at forty percent (40%) power, and the second set of ultra-quietpropellers may be operated at sixty percent (60%) power, e.g., to reducethe levels of noise emitted by the aerial vehicle during operation.

Upon reaching a second intervening waypoint, the aerial vehicle 810 isexpected to depart the coastal protected zone and travel over water forapproximately forty-seven minutes and a distance of approximatelythirty-nine miles. Over the water, where noise is typically not aconcern, the first set of high-speed propellers may be operated at onehundred percent (100%) power, and the second set of ultra-quietpropellers need not be operated. Finally, once the aerial vehicle 810reaches a third intervening waypoint (viz., on land), the aerial vehicle810 is expected to enter a residential zone, where suppressing noise maybe a primary concern. Within the residential zone, the first set ofhigh-speed propellers may be operated at ten percent (10%) power, andthe second set of ultra-quiet propellers may be operated at ninetypercent (90%) power, until the aerial vehicle 810 reaches thedestination.

Accordingly, an aerial vehicle, such as the aerial vehicle 810 of FIG.8, equipped with two or more discrete sets of propellers may beconfigured to operate in one or more distinct transit modes, which maybe selected on any basis, including but not limited to a position of theaerial vehicle, any operational characteristics of the aerial vehicle,or any environmental conditions encountered by the aerial vehicle duringflight. Moreover, one or more of the discrete propellers may be balancedor imbalanced, and may be specifically configured to emit sounds at aspecific sound pressure level or intensity and within a specificfrequency spectrum during operation. In this regard, the aerial vehiclemay operate in nearly any environment, and may be configured to emitvarious sounds (e.g., sounds at specific sound pressure levels or withinspecific frequency spectrums) that may be desired or appropriate for agiven environment.

Referring to FIG. 9, a block diagram of components of one system 900 foractive airborne noise abatement in accordance with embodiments of thepresent disclosure. The system 900 of FIG. 9 includes an aerial vehicle910, a testing facility 970 and a data processing system 980 connectedto one another over a network 990. Except where otherwise noted,reference numerals preceded by the number “9” shown in the block diagramof FIG. 9 indicate components or features that are similar to componentsor features having reference numerals preceded by the number “8” shownin FIG. 8, by the number “6” shown in FIG. 6, by the number “4” shown inFIG. 4A or FIG. 4B, by the number “2” shown in FIGS. 2A through 2I or bythe number “1” shown in FIGS. 1A and 1B.

The aerial vehicle 910 includes a processor 912, a memory 914 and atransceiver 916, as well as one or more propellers 920A, one or moremotors 960A for rotating the propellers 920A under power, and aplurality of sensors 965A (e.g., environmental or operational sensorsand/or sound sensors).

The processor 912 may be configured to perform any type or form ofcomputing function, including but not limited to the execution of one ormore machine learning algorithms or techniques. For example, theprocessor 912 may control any aspects of the operation of the aerialvehicle 910 and the one or more computer-based components thereon,including but not limited to the transceiver 916, the motor 960A, or thesensors 965A. The aerial vehicle 910 may likewise include one or morecontrol systems (not shown) that may generate instructions forconducting operations thereof, e.g., for operating the motor 960A or oneor more rudders, ailerons, flaps or other control components providedthereon (not shown). For example, where the propeller 920A includes oneor more computer-controlled features for imbalancing a balancedpropeller, or for varying a degree of imbalance of a propeller, e.g.,one or more of the adjustable covers 234D, 244D of the propeller 220D ofFIG. 2D, the processor 912 may control the operation of such covers234D, 244D directly or indirectly through one or more of controlsystems. Such control systems may be associated with one or more othercomputing devices or machines, such as the processor 912, and maycommunicate with the testing facility 970 and/or data processing system980 or one or more other computer devices (not shown) over the network990, as indicated by line 918, through the sending and receiving ofdigital data.

The aerial vehicle 910 further includes one or more memory or storagecomponents 914 for storing any type of information or data, e.g.,instructions for operating the aerial vehicle 910, information or datacaptured by one or more of the sensors 965A, or information or dataregarding propellers of various sizes, shapes or configurations and thenoises emitted thereby during operation. The transceiver 916 may beconfigured to enable the aerial vehicle 910 to communicate through oneor more wired or wireless means, e.g., wired technologies such asUniversal Serial Bus (or “USB”) or fiber optic cable, or standardwireless protocols such as Bluetooth® or any Wireless Fidelity (or“WiFi”) protocol, such as over the network 990 or directly.

The propeller 920A may be one or more bladed mechanical devices forgenerating one or more propulsive forces, e.g., lift and/or thrust, forthe aerial vehicle 910. The propeller 920A may have any mass ordimensions, or any number of blades, and may be balanced staticallyand/or dynamically, or imbalanced. The propeller 920A is coupled to themotor 960A, e.g., by a shaft. The motor 960A may be any type or form ofmotor, including but not limited to a brushless direct current (or DC)electric motor such as an outrunner brushless motor or an inrunnerbrushless motor. The motor 960A may receive instructions for operationvia one or more computer devices, e.g., the processor 912, or one ormore control systems (not shown) that may generate instructions forinitiating or stopping operations of the motor 960A, or operating themotor 960A at any predetermined speed.

The sensors 965A may include any type or form of sensor for capturinginformation or data regarding any aspect of the operation of the aerialvehicle 910 in general, or the motor 960A and/or the propeller 920A inparticular. For example, the sensors 965A may include one or morecomponents or features for determining one or more attributes of anenvironment in which the aerial vehicle 910 is operating, or may beexpected to operate, including extrinsic information or data orintrinsic information or data. Some such sensors 965A may include, butare not limited to, a Global Positioning System (“GPS”) receiver orsensor, a compass, a speedometer, an altimeter, a thermometer, abarometer, a hygrometer, or a gyroscope. Those of ordinary skill in thepertinent arts will recognize that the sensors 965A may further includeany type or form of device or component for determining an environmentalcondition within a vicinity of the aerial vehicle 910 in accordance withthe present disclosure. For example, the sensors 965A may also includeone or more air monitoring sensors (e.g., oxygen, ozone, hydrogen,carbon monoxide or carbon dioxide sensors), infrared sensors, ozonemonitors, pH sensors, magnetic anomaly detectors, metal detectors,radiation sensors (e.g., Geiger counters, neutron detectors, alphadetectors), attitude indicators, depth gauges, accelerometers or thelike, as well as one or more imaging devices (e.g., digital cameras).

The sensors 965A may also include other components or features fordetecting and capturing sound energy in a vicinity of an environment inwhich the aerial vehicle 910 is operating, or may be expected tooperate. Such sensors 965A may include one or more microphones (e.g., atransducer such as a dynamic microphone, a condenser microphone, aribbon microphone or a crystal microphone configured to convert acousticenergy of any intensity and across any or all frequencies into one ormore electrical signals, and may include any number of diaphragms,magnets, coils, plates, or other like features for detecting andrecording such energy), piezoelectric sensors (e.g., sensors configuredto convert changes in pressure to electrical signals, including one ormore crystals, electrodes or other features), or vibration sensors.

The testing facility 970 may be configured to operate propellers in amanner that simulates actual in-flight operation and performance. Thetesting facility 970 may include one or more computer devices includinga processor 972, a memory 974 and a transceiver 976, as well as one ormore propellers 920B, one or more motors 960B for rotating thepropellers 920B under power, and a plurality of sensors 965B. Theprocessor 972, the memory 974 and the transceiver 976 may executefunctions or operate in a manner similar to those described above withregard to the processor 912, the memory 914 and the transceiver 916 ofthe aerial vehicle 910, and may communicate with the data processingsystem 980 or one or more other computer devices (not shown) over thenetwork 990, as indicated by line 978, through the sending and receivingof digital data.

The propeller 920B may also be, like the propeller 920A, one or morebladed mechanical devices for generating one or more propulsive forces,e.g., lift and/or thrust, in an experimental environment within thetesting facility 970. The propeller 920B may have any mass ordimensions, or any number of blades, and may be balanced staticallyand/or dynamically, or imbalanced. The propeller 920B is coupled, e.g.,by a shaft, to the motor 960B, which may be any type or form of motor,such as a brushless DC electric motor. The motor 960B may receiveinstructions for operation via one or more computer devices, e.g., theprocessor 972, or one or more control systems (not shown) that maygenerate instructions for initiating or stopping operations of the motor960B, or operating the motor 960B at any predetermined speed.

The sensors 965B may, like the sensors 965A of the aerial vehicle 910,include any type or form of sensor for capturing information or dataregarding any aspect of the operation of the testing facility 970, orthe motor 960B and/or the propeller 920B in particular. For example, thesensors 965B may include one or more components or features fordetermining one or more attributes of an environment within the testingfacility 970, including extrinsic information or data or intrinsicinformation or data. Those of ordinary skill in the pertinent arts willrecognize that the sensors 965B may include any type or form of deviceor component for determining an environmental condition within thetesting facility 970 in accordance with the present disclosure. Forexample, the sensors 965B may also include one or more air monitoringsensors (e.g., oxygen, ozone, hydrogen, carbon monoxide or carbondioxide sensors), infrared sensors, ozone monitors, pH sensors, magneticanomaly detectors, metal detectors, radiation sensors (e.g., Geigercounters, neutron detectors, alpha detectors), attitude indicators,depth gauges, accelerometers or the like, as well as one or more imagingdevices (e.g., digital cameras).

The sensors 965B may also include other components or features fordetecting and capturing sound energy within the testing facility 970.Such sensor 965B may include one or more microphones (e.g., a transducersuch as a dynamic microphone, a condenser microphone, a ribbonmicrophone or a crystal microphone configured to convert acoustic energyof any intensity and across any or all frequencies into one or moreelectrical signals, and may include any number of diaphragms, magnets,coils, plates, or other like features for detecting and recording suchenergy), piezoelectric sensors (e.g., sensors configured to convertchanges in pressure to electrical signals, including one or morecrystals, electrodes or other features), or vibration sensors.

The data processing system 980 includes one or more physical computerservers 982 having a plurality of databases 984 associated therewith, aswell as one or more computer processors 983 provided for any specific orgeneral purpose. For example, the data processing system 980 of FIG. 9may be independently provided for the exclusive purpose of receiving,analyzing or storing operational characteristics, environmentalconditions, acoustic signals or other information or data received fromthe aerial vehicle 910 or the testing facility 970 or, alternatively,provided in connection with one or more physical or virtual servicesconfigured to receive, analyze or store such operationalcharacteristics, environmental conditions, acoustic signals or otherinformation or data, as well as one or more other functions. The servers982 may be connected to or otherwise communicate with the databases 984and the processors 983. The databases 984 may store any type ofinformation or data, including but not limited to operationalcharacteristics, environmental conditions, acoustic signals or otherinformation or data. The servers 982 and/or the computer processors 983may also connect to or otherwise communicate with the network 990, asindicated by line 988, through the sending and receiving of digitaldata. For example, the data processing system 980 may include anyfacilities, stations or locations having the ability or capacity toreceive and store information or data, such as media files, in one ormore data stores, e.g., media files received from the aerial vehicle 910or the testing facility 970, or from one another, or from one or moreother external computer systems (not shown) via the network 990. In someembodiments, the data processing system 980 may be provided in aphysical location. In other such embodiments, the data processing system980 may be provided in one or more alternate or virtual locations, e.g.,in a “cloud”-based environment. In still other embodiments, the dataprocessing system 980 may be provided onboard one or more aerialvehicles, including but not limited to the aerial vehicle 910, or withinthe testing facility 970.

The network 990 may be any wired network, wireless network, orcombination thereof, and may comprise the Internet in whole or in part.In addition, the network 990 may be a personal area network, local areanetwork, wide area network, cable network, satellite network, cellulartelephone network, or combination thereof. The network 990 may also be apublicly accessible network of linked networks, possibly operated byvarious distinct parties, such as the Internet. In some embodiments, thenetwork 990 may be a private or semi-private network, such as acorporate or university intranet. The network 990 may include one ormore wireless networks, such as a Global System for MobileCommunications (GSM) network, a Code Division Multiple Access (CDMA)network, a Long Term Evolution (LTE) network, or some other type ofwireless network. Protocols and components for communicating via theInternet or any of the other aforementioned types of communicationnetworks are well known to those skilled in the art of computercommunications and thus, need not be described in more detail herein.

The computers, servers, devices and the like described herein have thenecessary electronics, software, memory, storage, databases, firmware,logic/state machines, microprocessors, communication links, displays orother visual or audio user interfaces, printing devices, and any otherinput/output interfaces to provide any of the functions or servicesdescribed herein and/or achieve the results described herein. Also,those of ordinary skill in the pertinent art will recognize that usersof such computers, servers, devices and the like may operate a keyboard,keypad, mouse, stylus, touch screen, or other device (not shown) ormethod to interact with the computers, servers, devices and the like, orto “select” an item, link, node, hub or any other aspect of the presentdisclosure.

The aerial vehicle 910, the testing facility 970 or the data processingsystem 980 may use any web-enabled or Internet applications or features,or any other client-server applications or features including E-mail orother messaging techniques, to connect to the network 990, or tocommunicate with one another, such as through short or multimediamessaging service (SMS or MMS) text messages. For example, the aerialvehicle 910 and/or the testing facility 970 may be adapted to transmitinformation or data in the form of synchronous or asynchronous messagesto the data processing system 980 or to any other computer device inreal time or in near-real time, or in one or more offline processes, viathe network 990. Those of ordinary skill in the pertinent art wouldrecognize that the aerial vehicle 910, the testing facility 970 or thedata processing system 980 may operate any of a number of computingdevices that are capable of communicating over the network, includingbut not limited to set-top boxes, personal digital assistants, digitalmedia players, web pads, laptop computers, desktop computers, electronicbook readers, and the like. The protocols and components for providingcommunication between such devices are well known to those skilled inthe art of computer communications and need not be described in moredetail herein.

The data and/or computer executable instructions, programs, firmware,software and the like (also referred to herein as “computer executable”components) described herein may be stored on a computer-readable mediumthat is within or accessible by computers or computer components such asthe processor 912, the processor 972 or the processor 983, or any othercomputers or control systems utilized by the aerial vehicle 910, thetesting facility 970 or the data processing system 980, and havingsequences of instructions which, when executed by a processor (e.g., acentral processing unit, or “CPU”), cause the processor to perform allor a portion of the functions, services and/or methods described herein.Such computer executable instructions, programs, software, and the likemay be loaded into the memory of one or more computers using a drivemechanism associated with the computer readable medium, such as a floppydrive, CD-ROM drive, DVD-ROM drive, network interface, or the like, orvia external connections.

Some embodiments of the systems and methods of the present disclosuremay also be provided as a computer-executable program product includinga non-transitory machine-readable storage medium having stored thereoninstructions (in compressed or uncompressed form) that may be used toprogram a computer (or other electronic device) to perform processes ormethods described herein. The machine-readable storage media of thepresent disclosure may include, but is not limited to, hard drives,floppy diskettes, optical disks, CD-ROMs, DVDs, ROMs, RAMs, erasableprogrammable ROMs (“EPROM”), electrically erasable programmable ROMs(“EEPROM”), flash memory, magnetic or optical cards, solid-state memorydevices, or other types of media/machine-readable medium that may besuitable for storing electronic instructions. Further, embodiments mayalso be provided as a computer executable program product that includesa transitory machine-readable signal (in compressed or uncompressedform). Examples of machine-readable signals, whether modulated using acarrier or not, may include, but are not limited to, signals that acomputer system or machine hosting or running a computer program can beconfigured to access, or including signals that may be downloadedthrough the Internet or other networks.

Although the disclosure has been described herein using exemplarytechniques, components, and/or processes for implementing the systemsand methods of the present disclosure, it should be understood by thoseskilled in the art that other techniques, components, and/or processesor other combinations and sequences of the techniques, components,and/or processes described herein may be used or performed that achievethe same function(s) and/or result(s) described herein and which areincluded within the scope of the present disclosure.

For example, although some of the embodiments disclosed herein referencethe use of unmanned aerial vehicles to deliver payloads from warehousesor other like facilities to customers, those of ordinary skill in thepertinent arts will recognize that the systems and methods disclosedherein are not so limited, and may be utilized in connection with anytype or form of aerial vehicle (e.g., manned or unmanned) having fixedor rotating wings and having any intended industrial, commercial,recreational or other use. In particular, although some of theembodiments disclosed herein reference balanced or imbalanced propellershaving two blades, or aerial vehicles having four propellers, those ofordinary skill in the pertinent arts will recognize that the systems andmethods of the present disclosure may be utilized in connection withpropellers having any number of blades, and in connection with aerialvehicles having any number of propellers. Moreover, although some of theembodiments disclosed herein reference the use of balanced or imbalancedpropellers on aerial vehicles, those of ordinary skill in the pertinentarts will recognize that the systems and methods of the presentdisclosure may be utilized in connection with seagoing vessels, as well.

Furthermore, those of ordinary skill in the pertinent arts willrecognize that the systems and methods disclosed herein may be used tocause an aerial vehicle to radiate a series of sounds at predeterminedsound pressure levels and/or within predetermined frequency spectrums.By controlling the operation of a plurality of propellers, e.g., one ormore balanced or imbalanced propellers, an aerial vehicle mayeffectively emit music in accordance with one or more predeterminedscores, or may even synthesize speech.

It should be understood that, unless otherwise explicitly or implicitlyindicated herein, any of the features, characteristics, alternatives ormodifications described regarding a particular embodiment herein mayalso be applied, used, or incorporated with any other embodimentdescribed herein, and that the drawings and detailed description of thepresent disclosure are intended to cover all modifications, equivalentsand alternatives to the various embodiments as defined by the appendedclaims. Moreover, with respect to the one or more methods or processesof the present disclosure described herein, including but not limited tothe processes represented in the flow charts of FIG. 3, 5 or 7, ordersin which such methods or processes are presented are not intended to beconstrued as any limitation on the claimed inventions, and any number ofthe method or process steps or boxes described herein can be combined inany order and/or in parallel to implement the methods or processesdescribed herein. Also, the drawings herein are not drawn to scale.

Conditional language, such as, among others, “can,” “could,” “might,” or“may,” unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey in apermissive manner that certain embodiments could include, or have thepotential to include, but do not mandate or require, certain features,elements and/or steps. In a similar manner, terms such as “include,”“including” and “includes” are generally intended to mean “including,but not limited to.” Thus, such conditional language is not generallyintended to imply that features, elements and/or steps are in any wayrequired for one or more embodiments or that one or more embodimentsnecessarily include logic for deciding, with or without user input orprompting, whether these features, elements and/or steps are included orare to be performed in any particular embodiment.

Disjunctive language such as the phrase “at least one of X, Y, or Z,” or“at least one of X, Y and Z,” unless specifically stated otherwise, isotherwise understood with the context as used in general to present thatan item, term, etc., may be either X, Y, or Z, or any combinationthereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is notgenerally intended to, and should not, imply that certain embodimentsrequire at least one of X, at least one of Y, or at least one of Z toeach be present.

Unless otherwise explicitly stated, articles such as “a” or “an” shouldgenerally be interpreted to include one or more described items.Accordingly, phrases such as “a device configured to” are intended toinclude one or more recited devices. Such one or more recited devicescan also be collectively configured to carry out the stated recitations.For example, “a processor configured to carry out recitations A, B andC” can include a first processor configured to carry out recitation Aworking in conjunction with a second processor configured to carry outrecitations B and C.

Language of degree used herein, such as the terms “about,”“approximately,” “generally,” “nearly” or “substantially” as usedherein, represent a value, amount, or characteristic close to the statedvalue, amount, or characteristic that still performs a desired functionor achieves a desired result. For example, the terms “about,”“approximately,” “generally,” “nearly” or “substantially” may refer toan amount that is within less than 10% of, within less than 5% of,within less than 1% of, within less than 0.1% of, and within less than0.01% of the stated amount.

Although the invention has been described and illustrated with respectto illustrative embodiments thereof, the foregoing and various otheradditions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present disclosure.

What is claimed is:
 1. A method for operating an aerial vehicle, whereinthe method comprises: identifying information regarding at least oneattribute of a mission for the aerial vehicle having a first motor and asecond motor; rotatably coupling a first propeller to a first shaft ofthe first motor, wherein the first propeller is statically balanced anddynamically balanced; predicting a noise to be emitted during a rotationof a second propeller above a critical speed of the second propeller,wherein the second propeller is statically balanced and dynamicallybalanced; determining that at least one of a frequency spectrum of thenoise or a sound pressure level of the noise is not consistent with theat least one attribute of the mission; executing a modification to thesecond propeller, wherein the modified second propeller is at least oneof statically imbalanced or dynamically imbalanced after executing themodification; rotating the modified second propeller above a criticalspeed of the modified second propeller; determining that a noise emittedduring the rotation of the modified second propeller above the criticalspeed is consistent with the at least one attribute of the mission;rotatably coupling the modified second propeller to a second shaft ofthe second motor; and causing the aerial vehicle to perform the mission,wherein the at least one attribute includes at least one of: a locationof an origin for the mission; a location of a destination for themission; a location of an intervening waypoint between the origin andthe destination; at least one course to be traveled during theperformance of the mission; at least one air speed of the aerial vehiclerequired during the performance of the mission; or a mass of a payloadto be carried by the aerial vehicle during the mission.
 2. The method ofclaim 1, wherein a first center of mass is aligned along an axis ofrotation along the first shaft, wherein centrifugal forces acting uponeach of a first plurality of blades of the first propeller are equal toand counteract one another when the first propeller is rotated above thecritical speed of the first propeller, wherein a second center of massof the modified second propeller is not aligned along an axis ofrotation of the second shaft, and wherein centrifugal forces acting uponeach of a second plurality of blades of the modified second propellerare not equal to or do not counteract one another when the modifiedsecond propeller is rotated above the critical speed of the modifiedsecond propeller.
 3. The method of claim 1, wherein causing the aerialvehicle to perform the emission comprises: causing a rotation of thefirst propeller above a critical speed of the first propeller;determining a position of the aerial vehicle; determining that theposition of the aerial vehicle is within a predetermined range of one ofthe location of the origin, the location of the destination or thelocation of the intervening waypoint; in response to determining thatthe position of the aerial vehicle is within a predetermined range ofone of the location of the origin, the location of the destination orthe location of the intervening waypoint, stopping the rotation of thefirst propeller; and causing a rotation of the modified second propellerabove the critical speed of the modified second propeller.
 4. The methodof claim 1, wherein the second propeller comprises a first blade and asecond blade, and wherein executing the modification to the secondpropeller comprises at least one of: removing a core from the secondblade; inserting a slug into the second blade, wherein a density of theslug is greater than a density of a material from which the second bladeis formed; drilling at least a first hole in the second blade; orexposing, by a retractable cover disposed within the second blade, atleast a second hole in the second blade; or modifying at least one of ablade angle or a rake angle of the second blade.
 5. A method comprising:prior to an operation of an aerial vehicle, predicting an attribute ofthe aerial vehicle during the operation, wherein the aerial vehiclecomprises a first motor and a second motor; coupling a first propellerto a first shaft of the first motor, wherein the first propeller isstatically balanced and dynamically balanced; determining a modificationto a second propeller based at least in part on the predicted attribute;modifying the second propeller in accordance with the determinedmodification, wherein the second propeller is statically balanced anddynamically balanced prior to modifying the second propeller inaccordance with the determined modification, and wherein the secondpropeller is at least one of statically imbalanced or dynamicallyimbalanced after modifying the second propeller in accordance with thedetermined modification; coupling the modified second propeller to asecond shaft of the second motor; and initiating the operation of theaerial vehicle, wherein the operation of the aerial vehicle comprises:rotating the first propeller above a critical speed at a first time; androtating the modified second propeller above a critical speed at asecond time.
 6. The method of claim 5, wherein a center of mass of themodified second propeller is not aligned with an axis of rotation of thesecond shaft after the second propeller has been coupled to the secondshaft.
 7. The method of claim 5, wherein the predicted attribute is oneof: a predicted position of the aerial vehicle during the operation; apredicted course of the aerial vehicle during the operation; a predictedair speed of the aerial vehicle during the operation; a predictedenvironmental condition around the aerial vehicle during the operation;a predicted operating condition of the aerial vehicle during theoperation; or a predicted sound emitted by the aerial vehicle during theoperation.
 8. The method of claim 5, further comprising: prior tomodifying the second propeller, rotating the second propeller above thecritical speed; capturing information regarding a first sound emitted bythe second propeller while rotating above the critical speed, whereinthe information regarding the first sound comprises at least one of afirst frequency spectrum of the first sound or a first sound pressurelevel of the first sound; determining that at least one of the firstfrequency spectrum or the first sound pressure level is not desiredbased at least in part on the predicted attribute of the aerial vehicleduring the operation; and determining the modification based at least inpart on the first frequency spectrum or the first sound pressure level.9. The method of claim 8, wherein determining that the at least one ofthe first frequency spectrum or the first sound pressure level is notacceptable based at least in part on the predicted attribute of theaerial vehicle during the operation comprises: identifying informationregarding a second sound based at least in part on the predictedattribute of the aerial vehicle during the operation, wherein theinformation regarding the second sound comprises at least one of asecond frequency spectrum of the second sound or a second sound pressurelevel of the second sound, and wherein at least one of the secondfrequency spectrum or the second sound pressure level is desired basedat least in part on the predicted attribute of the aerial vehicle duringthe operation; and identifying a difference between the first sound andthe second sound based at least in part on the information regarding thefirst sound and the information regarding the second sound, wherein themodification is determined based at least in part on the difference. 10.The method of claim 8, further comprising: after modifying the secondpropeller, rotating the modified second propeller above the criticalspeed; capturing information regarding a second sound emitted by themodified second propeller while rotating above the critical speed,wherein the information regarding the second sound comprises at leastone of a second frequency spectrum of the second sound or a second soundpressure level of the second sound; and determining that at least one ofthe second frequency spectrum or the second sound pressure level isacceptable based at least in part on the predicted attribute of theaerial vehicle during the operation, wherein the modified secondpropeller is coupled to the second shaft in response to determining thatthe at least one of the second frequency spectrum or the second soundpressure level is acceptable.
 11. The method of claim 5, wherein thesecond propeller comprises a hub having a mounting bore, a first blademounted about the hub and a second blade mounted about the hub, andwherein modifying the second propeller in accordance with the determinedmodification comprises: modifying the first blade in accordance with thedetermined modification; and not modifying the second blade.
 12. Themethod of claim 11, wherein the determined modification is a reductionin at least one of a length or a width of at least one blade of thesecond propeller, and wherein modifying the first blade in accordancewith the determined modification comprises: reducing at least one of alength or a width of the first blade.
 13. The method of claim 11,wherein the determined modification is an increase in mass of at leastone blade of the second propeller, and wherein modifying the first bladein accordance with the determined modification comprises: inserting aslug into the first blade at a predetermined distance from the mountingbore of the hub, wherein a density of the slug exceeds a density of amaterial from which the first blade is formed.
 14. The method of claim11, wherein the determined modification is a reduction in mass of atleast one blade of the second propeller, and wherein modifying the firstblade in accordance with the determined modification comprises: removinga core from the first blade at a predetermined distance from themounting bore of the hub.
 15. The method of claim 11, wherein thedetermined modification is a reduction in mass of at least one blade ofthe second propeller, and wherein modifying the first blade inaccordance with the determined modification comprises: opening a holethrough at least a portion of the first blade at a predetermineddistance from the mounting bore of the hub.
 16. The method of claim 5,further comprising: determining an actual attribute of the aerialvehicle during the operation of the aerial vehicle at the first time;determining that the actual attribute of the aerial vehicle isconsistent with the predicted attribute; and in response to determiningthat the actual attribute of the aerial vehicle is consistent with thepredicted attribute, stopping the first propeller; and rotating themodified second propeller above the critical speed at the second time,wherein the actual attribute is one of: an actual position of the aerialvehicle during the operation; an actual course of the aerial vehicleduring the operation; an actual air speed of the aerial vehicle duringthe operation; an actual environmental condition around the aerialvehicle during the operation; an actual operating condition of theaerial vehicle during the operation; or an actual sound emitted by theaerial vehicle during the operation.
 17. A method comprising: rotating apropeller above a critical speed, wherein the propeller comprises atleast two blades mounted about a hub; capturing information regarding anobserved noise emitted by the propeller while rotating above thecritical speed, wherein the information regarding the observed noisecomprises at least one of a frequency spectrum of the observed noise ora sound pressure level of the observed noise; identifying informationregarding a desired noise, wherein the information regarding the desirednoise comprises at least one of a frequency spectrum of the desirednoise or a sound pressure level of the desired noise; determining thatthe observed noise is not consistent with the desired noise based atleast in part on the information regarding the observed noise and theinformation regarding the desired noise; in response to determining thatthe observed noise is not consistent with the desired noise, identifyingat least one difference between the observed noise and the desirednoise; determining an adjustment to one of the blades of the propellerbased at least in part on the at least one difference; modifying the oneof the blades of the propeller in accordance with the adjustment;rotatably coupling the modified propeller to a motor of an aerialvehicle; and causing the aerial vehicle to travel at a desired altitude,along a desired course, at a desired air speed or to a desired location,wherein causing the aerial vehicle to travel at the desired altitude,along the desired course, at the desired air speed or to the desiredlocation comprises: rotating the modified propeller above the criticalspeed.
 18. The method of claim 17, wherein the propeller is staticallybalanced and dynamically balanced prior to modifying the one of theblades of the propeller in accordance with the adjustment, and whereinthe modified propeller is at least one of statically imbalanced ordynamically imbalanced.
 19. The method of claim 17, wherein theadjustment is one of: drilling a hole in the one of the blades of thepropeller; removing a core from the one of the blades of the propeller;inserting a slug into the one of the blades of the propeller; changing arake angle of the one of the blades of the propeller; changing a bladeangle of the one of the blades of the propeller; shortening a length ofthe one of the blades of the propeller; or reducing a width of the oneof the blades of the propeller.
 20. The method of claim 17, whereinidentifying the information regarding the desired noise comprises:identifying at least one noise restriction for the aerial vehicle,wherein the noise restriction for the aerial vehicle is associated withat least one of the desired altitude, the desired course, the desiredair speed or the desired location; and determining the at least one ofthe frequency spectrum of the desired noise or the sound pressure levelof the desired noise based at least in part on the at least one noiserestriction.